This invention relates to the field of methods to determine adhesion properties of coatings, and, more particularly, to high adhesion strength HVOF (High Velocity Oxygen-Fuel) thermal sprayed coating.
HVOF (High Velocity Oxygen Fuel) thermal sprayed coatings are often used to extend the service life of metal parts in harsh environment. In order to determine the viability of the coatings, it is necessary to determine the adhesive strength of the coating.
Therefore, a standard ASTM test (C633-79) has been developed for this purpose. In this standard test, termed Tensile Adhesion Test, a coated specimen is attached to a support fixture of a tensile testing machine by epoxy. A tensile force is then applied by the machine until the coating fails. The stress at failure is called the tensile adhesion strength or bond strength.
However, some of the HVOF coatings exhibit higher adhesion strength than epoxy glue such that the standard test leads to delamination of the epoxy instead of the coating.
Therefore, there is a need for a testing method that will determine the adhesion strength of coatings that exceed the capability of the ASTM C633-79 standard test.
The present invention is a technique that determines the adhesion strength of any HVOF-coating/substrate system by spherical indentation. We emphasize that while we have characterized HVOF coating/substrate system, and demonstrated the test for this case, the test method is generally applicable to all coatings, regardless of the method used to obtain the coating. The principle of the method is illustrated in FIG. 1. The coated part is subjected to a loading-unloading cycle using a spherical indentor. During the loading phase, the substrate deforms plastically and during the unloading phase, the elastic-plastic mismatch between coating and substrate material properties gives rise to a tensile stress normal to the interface that will lead to delamination for a sufficient indentation load. A hybrid numerical-experimental approach is used to calculate the interfacial adhesion; the threshold load for delamination is found experimentally and a finite element model is used to calculate the corresponding adhesion stress for delamination.
The present invention enables the quantitative evaluation of the adhesion for any coating/substrate interface, regardless of bond strength, while the prior art can only evaluate interface strengths of a bond strength less than that of the adhesive used for bonding the two ASTM samples.